The purpose of this study is to examine hormonal rhythms in maturing male and female rats in order; 1) to establish when the rat develops the capacity to use time-of-day signals in the initiation and maintenance of neuroendocrine reflexes, and 2) to evaluate the consequences of the emergence of biorhythms in the timing of sexual maturation. The three hypothesis that will be tested are that; a) the rat begins to use photoperiod to set the timing of neuroendocrine reflexes at around 26 days of age; b) one consequnce of this development is that there is a prolactin surge at 28-30 days of age which plays a role in the timing of puberty onset and the coordination of synchronous waves of follicle development and estrogen secretion with a time course similar to that of the adult ovulatory cycle. It is proposed that puberty onset and the coordination of synchronous waves of follicle development and estrogen secretion with a time course similar to that of the adult ovulatory cycle. It is proposed that puberty is triggered by an ovarian signal that in turn depends upon the activation of the ovaries by a train of events set into motion by the emergence of the capacity of the animal to respond to time of day information. It is also proposed that a similar sequence of events controls the timing of puberty in males.